In cellular communication, in order to perform a communication, a user equipment located in a cell accesses a base station, receives control information for exchanging data from the base station, and is then able to exchange the data with the base station. In particular, since a user equipment transceives data through a base station, in order for one cellular user equipment to transmit data to another cellular user equipment, the former cellular user equipment transmits its data to the base station and the base station having received the data then transmits the received data to the latter cellular user equipment. Thus, in order for one user equipment to transmit data to another user equipment, since the former user equipment can transmit the data through the base station only, the base station performs scheduling on a channel and resource for the data transceiving and should transmit the scheduled information to each of the user equipment. Thus, in order to perform a communication between user equipment through a base station, the base station needs to allocate channel and resource for transceiving data to each of the user equipment. However, device-to-device communication has a structure to enable a user equipment to directly transceive data with a target user equipment without using a base station or relay node.
If device-to-device (D2D) communication for transceiving data in direct between user equipment is performed in a manner of sharing resources with an existing cellular network mentioned in the above description, each user equipment proceeds with the D2D communication after completion of resource allocation for the D2D communication. However, a method of transmitting an initial signal for initiating D2D communication has not been defined yet. Moreover, if the D2D communication for transceiving data in direct between user equipment is performed in a manner of sharing resources with the existing cellular network mentioned in the above description, it may cause a problem, to a user equipment currently performing the D2D communication, that synchronization with a user equipment currently performing D2D communication with the cellular network is mismatched or a problem that synchronization between user equipment currently performing D2D communications with different user equipment is mismatched.